Component casting

ABSTRACT

A deflector element  3,13  is provided, in order to improve temperature interface control with respect to directional solidification of a component casting. This deflector element  3,13  is formed during the mould making process and is arranged to be positioned with respect to a part  6  of a principal mould formation  2,12  within which the final cast component is produced. The deflector element  3,13  ensures a uniform temperature throughout the component cross-section relative to other parts of the component as it is solidified.

[0001] The present invention relates to component casting and moreparticularly but not exclusively to component casting of directionalsolidification or single crystal components for engines such as blades,seal segments and nozzle guide vanes.

[0002] Component casting is used in order to produce a wide range ofcomponents and members. Essentially, the component is cast in a mouldfrom a molten liquid and then allowed to cool in order to leave asolidified component. Some components such as turbine blades for jetengines require structural abilities such as high temperature creepresistant. This is achieved with turbine blades through forming a singlecrystal. At high temperatures, typically above half the absolute meltingtemperature of the metal, the grain boundaries become weaker than thegrain bodies such that the absence of such grain boundaries in a singlecrystal provides resistance to creep.

[0003] Techniques for producing single crystal components are wellknown. Essentially, the component is cast in a mould and then graduallywithdrawn from the furnace in an appropriate manner such thatpropagation of a single crystal is achieved. Typically, a so-called“pig-tail” selector is used in order to initiate a single grain orcrystal growth. The most important consideration with respect tocontinued propagation of a single crystal within the component is toensure so-called directional solidification. This is achieved by gradualwithdrawal of the component from the furnace such that the temperaturegradient is effectively controlled. Generally, the interface temperaturebetween the solid and liquid must be slightly lower than the meltingpoint of the solid and the liquid temperature must increase beyond theinterface. To achieve this temperature gradient, the latent heat ofsolidification must be conducted through the solidifying solid crystal.In any event, ideally the temperature interface should be flat andgradually progress through the component in order to ensure a uniformsingle crystal is provided with few, if any, defects at the interface.It should also be understood that the solidus/liquidus mix or mushy zonebetween the solid component and the liquid material should be renderedas stagnant as possible. Unfortunately, most components by their natureare shaped and so provide differing radiation heat effects due to thevarying thickness of the component at particular points. These changesrender it difficult to fully control the temperature gradient andtherefore an unacceptable proportion of components are rejected due todefects formed during casting.

[0004] A preferred method of component casting is that known as the lostwax process. This is a traditional technique in which a component isinitially formed as a wax structure and then a ceramic coat placed uponthat wax structure and allowed to harden. The wax is then removedtypically by heating in order to leave the ceramic as a mould for thecomponent. As indicated above, the component is cast from a moltenliquid and then allowed to cool and solidify.

[0005] In accordance with the present invention there is provided amethod of component casting comprising forming a mould with a displaceddeflector element adjacent to a part of a principal mould formationwhereby that deflector element controls the rate of heat loss from thatpart of the principal mould formation for more equalised solidificationthrough the component.

[0006] Preferably, forming of the mould is by a lost wax technique.

[0007] Normally, the displaced deflector element is horizontal.Possibly, the displaced deflector element is secured to a downpole ofthe mould.

[0008] Possibly, the deflector element is coated for improved radiationwith a low emission reflective material. Typically, such low emissionmaterial is a magnesium oxide.

[0009] Also in accordance with the present invention there is provided amould for component casting comprising a principal mould formation and adisplaced deflector element immediately adjacent a part of the principalmould formation to control in use the rate of heat loss from that partof the principal mould formation for more equalised solidification inuse through the component.

[0010] Possibly, the mould includes a downpole from which the displaceddeflector element is secured. Possibly, the displaced deflector elementis coated with a low, emission material such as a magnesium oxide.Typically, the displaced deflector element is located with sufficientgap to ensure there is sufficient mould thickness thus there is a gap ofapproximately 2 mm to 3 mm from the part of the principal mouldformation. Generally, the displaced deflector element has aconfiguration such that there is at least a 15 mm wide overlap with theprincipal mould formation.

[0011] Alternatively, the mould may be formed by stereo lithography.

[0012] An embodiment of the present invention will now be described byway of example only with reference to the accompanying drawings inwhich:

[0013]FIG. 1 is a side illustration of a quarter section of a waxprefrom of a cast component in accordance with the present invention;.

[0014]FIG. 2 is a front elevation of the wax perform depicted in FIG. 1;

[0015]FIG. 3 is a plan view in the direction A-A of the wax performdepicted in FIGS. 1 and 2;

[0016]FIG. 4 is a front view of a part of a mould in accordance with thepresent invention;

[0017]FIG. 5 is a plan view in the direction B-B of the part of themould depicted in FIG. 4; and,

[0018]FIG. 6 is a three quarter view, one component removed, of a mouldin accordance with the present invention.

[0019] As indicated previously, it is necessary to provide singlecrystal components for use in certain environments. For example, aturbine blade must be highly resistant to creep at high temperatures andso a single crystal with its lack of grain boundaries is the preferredstructure. In order to achieve such a single crystal structure closecontrol of solidification after casting is required. This close controlof temperature ensures as the component is removed from a furnace thatsingle crystal growth is propagated. Generally a calm flat. temperatureboundary or interface is required in order to achieve a single crystalstructure with no discontinuities.

[0020] One technique for forming a mould suitable for casting of asingle crystal component is using a lost wax process. Alternatively, themould could be formed by a stereo lithography process. A one part mouldis produced by coating or investing a wax replica structure of thedesired final cast component with a refractory slurry which then sets atroom temperature. The wax is then removed generally by melting in orderto leave a cavity in the refractory slurry which is a mould of exactlythe same shape as the desired component, that is to say the initial wax.structure. The cavity is then filled with a molten liquid material tocast the final component. In accordance with the desired single crystalstructure it is necessary then to appropriately arrange cooling of theliquid, that is to say solidification, in order to create that desiredsingle crystal structure.

[0021]FIGS. 1, 2 and 3 illustrate a wax form utilised in accordance withthe present invention. Thus, the form 1 comprises a principal mouldformation 2 equivalent to the shape of the desired cast component and adisplaced deflector element 3. The displaced deflector element 3 issecured to a downpole 4 which is also part of the form 1 and theeventual mould. As described previously, this mould structure is formedin wax and a quarter section is shown in FIGS. 1 to 3. Thus, the.deflector 3 will generally be presented all round the principal mouldformation 2 at a desired position.

[0022] As indicated previously, temperature interface control withrespect to the solidification of the component is necessary in order toachieve the desired single crystal structure. A technique known asdirectional solidification is used whereby the mould incorporating thecooling cast component is gradually removed from a furnace in order toprecipitate the desired single crystal structure. Generally, a pig-tailselector 5 is used in order to initiate such single crystal structurepropagation. Unfortunately, as can be seen in the mould form 1 formed ofwax shown in FIGS. 1 to 3 and in particular the principal mouldformation 2 which is a replica of the final cast component, thedimensions and thicknesses of that cast component vary along its length.Such variations make it very difficult to achieve the desired flattemperature gradient interface in order to achieve the desired singlecrystal structure. In particular as depicted in FIGS. 1 to 3 rootportions of the principal mould formation 2 are generally narrower thanother parts of that formation 2 such that there can be a variabletemperature gradient through the cross-section of the formation 2 andtherefore cast component in these areas.

[0023] In accordance with the present invention the deflector element 3is positioned about the root part of the formation 2 whereby thetemperature gradient across the component at that position is maintainedas substantially flat. Thus, central parts of the cast component will beat approximately the same temperature as surface parts of that componentdespite the relatively thin cross-section of the component. In suchcircumstances, the whole width and cross-section of the component issubstantially at the same temperature and will cool at the same rate toensure better single crystal propagation or directional solidificationwithout defects.

[0024] The deflector element 3 is formed by providing upon the downpole4, during formation of the wax replica for the finished component, ashaped protrusion which extends towards the principal mould formation 2such that a surface 6 is adjacent but displaced from an overlap. zone ofthe principal mould formation 2. In accordance with the usual lost waxprocess the structure shown in FIGS. 1 to 3 is then coated with aceramic slurry in order to provide the structure as shown in FIGS. 4 and5. This comprises coating the wax with several layers of ceramic slurryand stucco and allowing those layers to solidify. The wax is thenremoved to leave the mould behind. Molten metal is then placed in themould and allowed to solidify in accordance with the control regime asdescribed previously in order to create a single crystal. This techniqueis known as single crystal or directional solidification (CDS).Directional solidification furnaces usually comprise one or two heaterzones, an insulation ring or baffle and a lower cooling zone. The mouldin which the casting is solidifying is slowly withdrawn from the heaterzone into the cooling zone at a predetermined rate in order to produceuni-axial heat flow in the opposite direction to the desired crystalgrowth. In such circumstances, it will be appreciated that variations inthe cross-sectional area of the component can create convectiveinstabilities, local thermal gradients and other problems which in turncan create defects within the crystal structure.

[0025] In accordance with the present invention and as depicted in FIGS.4 and 5, the deflector element 13 is designed to control the radiativeheat flux from the relatively narrow component part of the principalmould formation 12 cavity during solidification. This control isachieved by modifying the radiation view factors of local overlap areasof the principal mould formation 12. In particular, local areasspecifically on the inner or downpole 14 side of the principal mouldformation 12 where a traditional furnace baffle would be ineffective. Ascan be seen in the Figures the deflector element 3; 13 essentiallycomprises a radiation deflector in the form of a horizontal platemounted upon the downpole 4; 14 at a position below an identifiedtemperature gradient problem area of the casting. The deflector 3; 13 isadded during the wax form arrangement stage as a thin wax plate attachedto the down pole 4. Alternatively, the deflector 3; 13 may be a ceramicplate instead of a wax plate provided a sufficient mould thickness iscreated and it has the correct shape. The deflector 3; 13 ideally willfollow the contour of the opposed overlap part of the componentprincipal mould formation 2, 12 to avoid higher heat loss around thesurface of that part. It is possible to use a simple disc as thedeflector 3; 13 but this 30 would provide a lower benefit. Generally,the deflector 3; 13 is formed by the wax protrusion or plate will. beprocessed or modelled as a 2 mm to 3 mm thick plate of wax with anominal reflector thickness and a coating with the layer of ceramicslurry and stucco in the order of 15mm. The thickness can be between 1to 3 mm, a vertically thin feature is required, but thickness isgoverned by the strength of the wax used, again a gap of 15 mm is notspecific to the present invention. When the preferred loss wax techniqueis used such a gap allows a mould shell thickness to form about the waxpattern of sufficient strength for casting.

[0026] As indicated previously,. the effect of the deflector 3; 13 is toreduce the radiation view factor for the principal moulding formation 2;12 on the inner downpole, 4: 14 side of the casting within thatprincipal moulding formation 2; 12. In such circumstance, there is aheat flux from these protruding features which in turn balances thetemperature gradient at the critical solid/liquid interface. Thedeflector 3, 13 reflects heat back onto the protruding feature of thecasting (principal mould feature) so reducing the radiative heat lossfrom these features. A more controlled temperature gradient at thissolid liquid interface reduces the risk of secondary grain formationcaused by convective flow instabilities and other problems within thestill fluid casting material. The deflector 3, 13 also reduces otherdefects caused by low thermal gradients, such as linear eutectics in DScastings and shrinkage porosity. Thus, defects as a result of a numberof problems are relieved.

[0027] It will be appreciated by allowing more complex componentstructures to be used additional features can be cast into the componentrather than requiring those features to be achieved by subsequentmachining of the cast component. Clearly, such machining also requiresremoval of material which is wasteful and expensive. Such machining mayalso introduce stress fracturing which may be exploited by temperaturecycling in use of the component and so cause premature failure.

[0028] The present deflector 3, 13 is created during the wax formationstage and so unlike fixed baffles within a furnace can be placed at anylocation where there is an identified need. It will also be understoodthat the deflector 3, 13 need not follow the extreme outer profile ofthe mould so the reflector can be placed anywhere that there is aconsistent repeatable defect identified with variable overlap andspacing determined as necessary for temperature control. The deflector3; 13 can be easily produced upon the downpole 4 during the wax formfabrication 3; 13 stage or as an additional piece added to the downpole4 during such wax structure fabrication. Essentially, the deflector isformed by the already present wax and ceramic slurry/stucco combinationand in such circumstances does not significantly add to costparticularly in comparison with baffles and more sophisticated twin zoneheaters used in existing furnace arrangements in order to overcome thedescribed defect problems due to solid liquid interface temperaturevariations. The deflector could be made from any material as long as iteither does not react with the cast alloy, or can be removed duringdewax or pre-firing of the mould, can be shelled or coated to form themould, or can be produced as a. feature in a stereo lithographic ceramicmould.

[0029]FIG. 6 illustrates a three quarter part front perspective of apractical mould 100 in accordance with the present invention, that is tosay with one component mould removed. Thus, it can be seen in thisembodiment that the mould in use presents four principal mouldformations 102 located substantially perpendicular to each other on abase plinth. However, the deflector can be used with any number ofcomponents on a mould but may be limited by the size of the component,or the size of the furnace used to cast the component. The deflectormust follow the outer profile of the component and is positioned in adesired location to prevent defects. The deflectors 103 are formed as adownpole 104 to be presented at appropriate positions for temperaturegradient stabilisation as described above.

[0030] It will be appreciated a number of deflectors in accordance withthe present invention could be utilised at different positions relativeto the principal moulding formation in order to ensure appropriatetemperature control during solidification of the casting. The specificshape, size and location of the reflector is determined through aradiative modelling procedure in order to best maintain temperature toachieve the consistency of temperature across the solid-liquid interfacethrough the component as it is gradually withdrawn from the furnace.

[0031] With a turbine blade, the present deflector will generally beused with respect to the thermal gradient in the root fir tree or shankneck regions of the casting which forms the turbine blade. As indicatedpreviously, these are the parts of the turbine blade where the reducedcross-sectional area can result in variable thermal gradient andconvection flow induced defects. Generally, root and shank parts are thebiggest problem areas on a turbine blade, but a deflector could be usedon any directional solidification or single crystal formed component.Deflector being located below any feature which is the source of asolidification defect.

[0032] In order to enhance the performance of the deflector it will beappreciated that once the layer of ceramic slurry and stucco has driedand hardened, an appropriate high emission material such as a magnesiumoxide paint may be applied to the deflector in order to further enhanceits radiative abilities. It will be understood that it is heat from thecasting within the principal mould formation which is reflected back tothat moulding in order to control temperature as required to resistgrain boundary formation. It will be understood that radiative heat lossis the main manner of cooling of the casting within the principalmoulding formation as the mould is generally within a vacuum furnacewhereby convective airflows about the mould are removed to again avoiddifferential cooling along the length of the casting within theprincipal mould formation. Nevertheless, generally the upper part of thefurnace and therefore the principal mould formation and casting will behotter than the lower part due to positioning of the heaters, etc. Thus,the mould is gradually removed from the furnace by a downward motionthus gradually cooling the casting in the principal moulding formation.

[0033] Although mainly described with reference to use of a downmpole itwill be understood that separate upstanding legs may be used in themould where space is available in order to appropriately present thedeflector element to the principal mould formation. Similarly, althoughdescribed with regard to a turbine blade other components such as a sealor nozzle vane could be made using the present method and/or mould.

[0034] Whilst endeavouring in the foregoing specification to drawattention to those features of the invention believed to be ofparticular importance it should be understood that the Applicant claimsprotection in respect of any patentable feature or combination offeatures hereinbefore referred to and/or shown in the drawings whetheror not particular emphasis has been placed thereon.

1. A method of component casting comprising forming a mould with adisplaced deflector element adjacent to a part of a principal mouldformation whereby the deflector element controls the rate of heat lossfrom that part of the principal mould formation.
 2. A method as claimedin claim 1 wherein the mould is formed by a lost wax process or stereolithographic process.
 3. A method as claimed in claim 1 or claim 2wherein the displaced deflector element is secured to a downpole of themould.
 4. A method as claimed in claim 1 or claim 2 wherein thedisplaced deflector element is secured to a separate upstanding leg topresent the deflector element to the principal mould formation.
 5. Amethod as claimed in any preceding claim wherein the deflector elementis coated for improved radiation with a low emission material.
 6. Amethod as claimed in claim 5 wherein the defector element is coated witha magnesium oxide coating.
 7. A mould for component casting comprising aprincipal mould formation and a displaced deflector element immediatelyadjacent a part of the principal mould formation to control in use therate of heat loss from that part of the principal mould formation.
 8. Amould as claimed in claim 7 wherein the mould includes a downpipe fromwhich the displaced deflector element is secured.
 9. A mould as claimedin claim 7 wherein the mould includes an upstanding leg to present thedeflector element to the principal mould formation.
 10. A mould asclaimed in any of claims 7 to 9 wherein the displaced deflector iscoated with a low emission material.
 11. A mould as claimed in claim 10wherein the low emission material is a magnesium oxide coating.
 12. Amould as claimed in any of claims 7 to 11 wherein the displaceddeflector element is approximately 2 mm to 3 mm thick.
 13. A mould asclaimed in any of claims 7 to 12 wherein the displaced deflector elementhas a configuration such that there is at least a 15 mm wide overlapwith the principal mould formation.
 14. A mould as claimed in any claims7 to 13 wherein the mould is formed from a ceramic material locatedabout a wax perform of a desired component casting.
 15. A method ofcomponent casting substantially as hereinbefore described with referenceto the accompanying drawings.
 16. A mould substantially as hereinbeforedescribed with reference to the accompanying drawings.
 17. A componentcasting formed by a method as claimed in any claims 1 to 6 or claim 15.18. A component casting formed using a mould as claimed in any of claims7 to 14 or claim
 16. 19. An engine incorporating a component castingformed by a method as claimed in any of claims 1 to 7 or claim
 15. 20.An engine incorporating a component casting formed using a mould asclaimed in any of claims 7 to 14 or claim
 16. 21. Any novel subjectmatter or combination including novel subject matter disclosed herein,whether or not within the scope of or relating to the same invention asan˜ of the preceding claims.